1. Field of the Invention
This invention is a reverse Stirling cycle heat pump.
2. The Prior Art
Heat pumps are now driven by electrically or engine driven compressors at relatively low total thermal efficiency. The reversed Rankine cycle heat pumps require refrigerants which are hostile to our environment. The substitutes proposed for the CFC based refrigerants are either very expensive or toxic or inflammable. Air system heat pumps based on a reversed Brayton cycle are relatively inefficient as are absorption heat pumps.
Reverse Sterling cycle heat pumps are capable of relatively high total thermal efficiency without the use of CFC's. A reverse Stirling cycle consists of a cooled isothermal-thermal compression, constant volume reversible cooling, isothermal-thermal expansion, and finally reversible constant-volume heating.
Available embodiments of Stirling cycle heat pumps operate with discontinuous, unsteady flow during four separate changes of state accomplished by a sequence of piston movements and the alternate movement of warmer and cooler gas in opposite directions through a heat storage matrix. All the Stirling cycle examples cited in the latest literature show discontinuous flow devices. This time consuming sequence results in a low rate of heat removal for a fixed size of equipment in a Stirling cycle heat pump and such units are expensive for the heat pumping rate achieved.
Constant flow, constant volume thermal compression with regenerative heat transfer as required for constant flow Stirling cycle function is considered impossible by Stirling cycle heat pump authorities as noted J. Wurm, J. A. Kinast, T. R. Roose, W. R. Staats in the recent publication, "Stirling and Vuillemier Heat Pumps," McGraw-Hill, 1991, state on page 24, "A recuperative heat exchanger that can achieve compression or expansion of a fluid at constant specific volume has not yet been invented. Regenerative heat exchange is also not possible because the regenerator would have to move from one flow stream to the other which seems impossible because the two streams have varying pressures." This invention solves these problems.
An air-only air-conditioner invented by Dr. Thomas C. Edwards noted. "Air-only air-conditioner surprises auto makers," Machine Design, Mar. 6, 1975, p. 10, has a vaned compressor and expander operating on the same slotted rotor as in one embodiment of the present invention. However, the unit lacks direct thermal contact with a heat sink or a heat source for isothermal operation. In addition, there is no integration, on the device nor on the system level, of a regenerative constant volume heat exchanging as in the present invention.
Kelly (U.S. Pat. No. 3,537,269) shows two rotors, one acts as a displacer, ". . . in a similar manner to that of the reciprocating displacer piston . . . " (Column 1, line 55). Thermal storage is required between cycle phases. (Column 2, lines 27 and 33). The alternate and repeated heating and cooling of filaments or other materials between cycles is inherently inefficient and gets worse with increased heat pump speed. EQU ln ( T, t/ T Initial)=-t/(RC THERM)
where RC THERM=.rho.cV/hA=THERMAL TIME CONSTANT and ( T, t)=Temperature difference between matrix and stream at time t.
However, cycle frequency=1/t
THEREFORE:
lN ( T, T/ T Initial)=+Freq (RC therm)
Thermal error, the failure of matrix material and gas stream, and consequently, the two streams, to approach the same temperature, which is a positive function of frequency, increases with frequency and heat pump speed. This theoretical prediction has been verified experimentally. The temperature difference between the heated stream and the cooled stream does increase with regenerator cycle frequency. (FIG. 11, K. Hamaguchi et al. "Effects of Generator Size Change", 26th Intersociety Energy Conversion Engineering Conference, August 1991, Volume 5, page 298).
The reheat loss, within the regenerator also increase with cycle frequency (FIG. 10 of ibid.). Therefore, heat exchanger effectiveness along with heat pump COP can be expected to drop with regeneration frequency and heat pump speed in alternately heated and cooled heat exchangers. In actual practice, this happens (FIG. 4a. Suganami et al., "Development of Small-Scale Stirling Engine Heat Pump System", 26th I.E.C.E.C., August 1991, Volume 5, page 264).
The reference selects air as the working gas which has a poor combination of heat transfer coefficient and specific heat to require a relatively large heat pump for the same capacity as hydrogen or helium (J. C. Daley, et al. "Stirling Engine Performance Optimization With Different Working Fluids "21st I.E.C.E.C., August 1986, Volume 1, page 275).
T. C. Edwards et al. (U.S. Pat. No. 4,494,386) teach a rotary compressor in a vapor--compression refrigeration system with a means of reducing friction between the vane tips and the stator wall. Functionally, there is no isothermal expansion, no regenerative heat transfer and no possibility of operating a Stirling cycle. Structurally, there is no integration of compressor, regenerator and expander in one enclosed device.
L. W. Midolo (U.S. Pat. No. 4,211,093) teaches a two-stage rotary compressor within one case. The sliding vane compressor drives a vapor-compression cooling system. There are no structural provisions or capability for a reverse Stirling cycle heat pump.
P. A. M. Leger (U.S. Pat. No. 3,487,424) teaches synchronized rotary displacers that "periodically" drive gas into hot and cold chambers and sequentially through a regenerator to achieve a reverse Stirling cycle heat pump. Valves are required to reverse the flow through a regenerator 7 wherein storage material is periodically heated and cooled and the gas flowing therein is alternately cooled and heated. In contrast, the present invention has no timed valves and no periodic reversing of flow and no thermal storage regenerators.
R. R. Hanson and E. A. Braden (U.S. Pat. No. 3,189,162) teach a rotary compressor to drive a vapor-compression space cooler. There are no provisions for any of the essential Stirling cycle heat pump processes. The structure has no heat exchanger for regeneration nor any capability to perform such function.